Since the early 1970s the increased cost of petroleum products has driven a growing interest in the recycling of asphalt paved road surfaces. It has become increasingly important to recycle in order to preserve these non-renewable resources and save cost.
An asphalt paved road surface is made up of a combination of graded aggregates (crushed rock and sand) and asphalt cement (a dark, sticky petroleum based adhesive) and air voids. These materials are typically blended together in a central plant, delivered to the roadway by trucks and spread and compressed onto the road surface.
It is well known that over time asphalt-paved road surfaces age and deteriorate for a number of reasons. Temperature fluctuations, precipitation, and UV exposure cause the pavement to lose its flexibility, which causes the surface to crack and deteriorate. Moreover, chemicals within the asphalt cement gradually dissipate or their properties change (harden and lose adhesive properties) further causing the eventual failure of the surface.
Originally, pavement recycling involved cold milling machines that were used to grind out an aged or damaged pavement which was then hauled back to a central processing plant where it would be heated and mixed with new material. The mixture would then be hauled back to the road site and be reinstalled back on the road surface. Generally speaking the cold grinding of aged or damaged pavement tends to fracture the aggregate requiring the selected addition of new aggregate material to compensate for the fractured aggregate.
Subsequently insitu processes for the recycling of asphalt have been developed. Some such processes involve heating and are frequently referred to as “hot in-place asphalt recycling” (hereinafter referred to as HIPAR).
HIPAR consists of many known methods and machines but generally it involves insitu heating the asphalt pavement to soften, loosening the softened pavement with scarifiers or grinders, adding and mixing in new asphalt mix and rejuvenating oils and then reinstalling the combined mixture back on the road surface at substantially the same grade elevation. It is important not to add too much new asphalt mix using this technique or the newly repaved lane will be too high in relation to the adjoining lane resulting in a safety hazard to motorists.
The prior art has evolved a number of techniques for carrying out HIPAR on asphalt road surfaces. Typically the prior art incorporates large infrared or hot air heaters, which heat the road surface to about 275to 350 F. When the pavement is heated to this temperature range it becomes softened enough to remove it without crushing the aggregates. Overheating the surface to greater temperatures can result in hardening and loss of adhesive properties of the asphalt cement. Moreover overheating result in excessive blue and black smoke emissions which not only damage the environment but also cause a safety hazard to the workers and motorists in the area.
When recycling a paved surface it is advantageous to achieve a depth of processing of least 1.5 to 2 inches in order to sufficient remove cracks and defects and prevent or delay their return. Due to the poor thermal conductivity of aged asphalt pavement achieving this depth without overheating has generally not been possible.
One technique that was developed comprised heating and processing the pavement in two or more stages (hereinafter referred to as Multi-Stage), which in one example consisted of heating and removing layers of 0.5 to 1 inch thickness per stage. The Multi-Stage technique overcame some of the previously mentioned challenges but faced new problems resulting from managing the asphalt removed from the first removed layer while heating and grinding the second layer. In the case of three and four stage machines this problem progressed beyond the first and second layers.
One prior art Multi-Stage technique of dealing with this problem is described U.S. Pat. No. 4,929,120 issued to Wiley and Rorison. A conveyor is used to carry the asphalt removed from the first layer over top of the subsequent heater. Although this patent represents an advance over the prior art, considerable capital cost and maintenance cost can be incurred for such a conveyor.
A second prior art method of dealing with this problem is described in U.S. Pat. No. 4,850,740 issued to Wiley. This patent describes a prior art method and apparatus whereby there is a longitudinal gap in the center of the second and subsequent heaters banks to allow the heated, windrowed asphalt removed from the first and subsequent layers to pass through without overheating. Again while this patent represents an additional advance over the prior art, issues can arise in that the existing paved surface beneath the gap in the subsequent heaters does not get heated sufficiently. This is partly due to the poor heat transfer from the hot windrow to the unground pavement surface below. The windrow is simply not hot enough to sufficiently heat the unground pavement surface below. In addition air voids present in the loosened asphalt in the windrow inhibit heat transfer to a sustained temperature right against the unground pavement below the windrow. The resulting lack of heating and softening in the area below the previous removed windrow causes the aggregates in this lower area to become fractured during milling which reduces the quality of final recycled asphalt product. Another problem encountered is that this asphalt cement within this unheated material does not become hot enough to become liquid or pliable and, therefore, it does not bind with the other material in the roadway or become mixed with the later added rejuvenators. Another issue to be considered is the cooling effect this unheated material has on the total final mixture. This means the other materials not in this area below the heated windrow must be heated to a higher temperature to compensate to achieve the desired temperatures for proper mixing in of rejuvenates and pressing (compacting) back on the road. This can result in overheating of the road surface damaging the asphalt cement and causing smoke emissions as mentioned previously.
It is an object of this invention to provide an improved method of rejuvenating an asphalt paved road surface and its associated apparatus.
It is an aspect of this invention to provide a method of rejuvenating an asphalt paved road surface comprising: grinding a portion of the road surface to produce a first ground or loosened asphalt portion; removing the first loosened asphalt portion to present a recess in the remaining portion of the road; heating the remaining portion of said road; grinding the heated remaining portion of the road to present a second loosened asphalt portion; gathering the second loosened asphalt portion on to the recess means and exposing a lower layer of remaining portion of the road; heating the lower layer of the remaining portion of the road; gathering the heated lower layer of the remaining portion of the road to present a third loosened asphalt portion; commingling the third loosened asphalt portion with the second loosened asphalt portion onto the recess means; introducing fresh asphalt to the commingled asphalt portion so as to repair the road.
It is another aspect of this invention to provide a method of rejuvenating an asphalt paved road surface comprising: grinding a first portion of the road surface to a selected depth and width to produce a first loosened asphalt portion; removing the first loosened asphalt portion to present a strip in the central region of the remaining portion of the road surface; heating the remaining portion of the road surface to a selected temperature and time duration; grinding the heated remaining portion of the road surface to a selected depth to present a second loosened asphalt portion; windrowing the second loosened asphalt portion onto the strip to expose a lower layer of the remaining portion of the road; heating the lower layer of the remaining portion of the road to a selected temperature and time duration; grinding the heated lower layer of the remaining portion of the road to a selected depth to present a third loosened asphalt portion; windrowing the third loosened asphalt portion on to the strip and commingling the third loosened asphalt portion with the second loosened asphalt portion; introducing fresh asphalt to the commingled asphalt portions so as to repair the road.
These and other objects and features of the invention shall now be described in relation to the drawings.